Aluminum alloy casting and process of making the same



Patented May 31, 1932 UNITED STATES PATENT oerce ALADAR PACZ, OFCLEVELAND, OHIO,- ASSIGNOR OF ONE-HALF TOALUMINUM COI- PANY OF AMERICA,01 PITTSBURGH, PENNSYLVANIA, A. CORPORATION OF PENN- SYLVANIA, ANDONE-HALF TO METALLGESELLSCHAFT AKTIENGESELLSCHAFT, OF

FRANKFORT-ON-TEE-MAIN, GERMANY,

A CORPORATION OF GERMANY.

AI UMINUM ALLOY CASTING AND PROCESS OF MAKING THE SAME No Drawing.Application filed February 24,

1928, Serial No. 256,820, and in- Germany larch 22, 1927.

Renewed.- March 25, 1932.

This invention relates primarily to the production of heat-treatedcastings of aluminum-base alloy, and its chief object is to provide amore economical and more flexible process for producing such articles.A'further object is to provide a method of accelerating theheat-treatment of aluminum alloy castings, whereby the expected resultsof heat-treatment, and in some cases even better results, can beobtained in less time than by prior processes. In some cases usefulresults can be obtained at lower temperatures. Another object is toprovide an improved article of the kind indicated, having high tensilestrength and elongation and increased hardness. To these and other endsthe in vention comprises the novel article and process hereinafterdescribed.

In the making of heat-treated castings of aluminum-copper alloys asheretofore carried out to obtain maximum results it has been necessaryto heat the castings for a prolonged period. For castings of small ormedium dimensions the minimum time was about seven hours, and for piecesof'larger dimensions the time required in practice was often as long astwenty hours. These long periods necessarily added to the cost ofmanufacture. It is also known that to develop the best pro erties of thecasting the temperature of the heat-treatment must be close to themelting point of the lowest-melting eutectic. This requirementdemandsaccurate control of temperature, which is a matter of somedifliculty in the case of long periods of heat-treatment, and need hastherefore been felt for a simpler, more economical and more flexiblemethod, which. can be carried out with ease and certainty, and this needmy present invention is designed to meet.

The invention is based on the discovery that the presence of a suitableamount of titanium in aluminum-base casting alloys containing copperaffects the susceptibility of the casting alloys to heat-treatment in away not hitherto recognized, despite the fact that the art containednumerous examples of aluminum-copper-titanium alloys for making articleswhich were to be worked before heat-treatment. No practical advantage,however, resulted from such suggestions, and indeed, so far as I havebeen able to learn, the titanium was not used for the purpose ofimproving or in any way modifying the heattreating properties of thealloy but only for hours and in some cases enabling lower temperaturesto be used, thus permitting wider variations of furnace temperatureswithout disadvantageous results. In fact as good results are possiblewith my invention in three hours heat-treatment with the best priorpractice in a period more than twice that long. Moreover, the resultsobtained by theuse of the titanium in my invention is to a large extentindependent of the pouring temperature and the thickness of the castsection, thus permitting the pouring temperature to be adjusted to suitcasting conditions without loss of desirable physical properties. tor ofgreat commercial importance.

According to my invention the ran 0 of titanium content is from 0.05five undredths) of .0116 per cent to 0.5 one-half) approximately, butthe preferred range is from about 0.1 per centto about 0.3 per cent;whereas the prior roposals for the use of titanium in alloys w 'ch wereto be worked before heat-treatment have called for about 1 per cent oftitanium or even more.

" gin preparing the alloy I have found it adof one per cent,

as could be obtained.

This latter phenomenon isa fee-- presenceof' an alkali metal fluoride.One

convenient method of doing this is to first prepare an aluminmn-titaniumalloy by the introduction of titanium oxide along with alumina in thewell-known method of pro- .atthee ducing aluminum by electrolyticreduction of alumina in a fused-bath of natural or artificial cryolite,so that titanium oxide is reduced also. If titanium bearing aluminaisavailable, it can be advantageously used for this purpose. The desiredcopper content can be su plied later by the addition of a suitable auminum-copper alloy containing sa from 20 to 50 per cent of copper. Anoter method of introducing titanium which I have found useful is to allowa double fluoride of titanium, such as sodium orpotassium-titaniumfluoride. to react with molten aluminum or molten aluminum-copper alloy.This method is more expensive, but has the advantage that it can becarried out in any foundry without the use of the electrolytic cell.Titanium may also be supplied in the .form of a copper titanium alloy,such as the cupro-titanium of commerce, which contains about 5 to 10 percent titanium. In the latter case the titanium is added without contactwith molten fluoride. While a substantial effect is secured by theintroduction of-titanium in this way, I have found in general thatbetter results are secured when the titanium is alloyed with thealuminum or the aluminum-copper alloy through the medium of or inassociation with alkali metal fluorides. Why this should be so, I amunable at present to state, but nevertheless I have found that methoddistinctly advantageous. In practice it has been found convenient toprepare first an aluminum-titanium alloy containing about 0.25 per centtitanium, and

. then make up the final alloy by adding in the proper amount analuminum-copper alloy rich in the latter metal. together with magnesium,manganese,'-or any other constituent that may be desired in, thefinished casting.

The copper content of the alloy is preferably held between about 3 and5.51per cent/- If a. greater proportion of copper-is em-- ployed, as forexample to increase the hardness and raise the yield point of thecasting, nse, perhaps. of ductility, it; will be dificu t if notimpossible to dissolvelthe excess copper constituent (CuAl,) by heating;Even under these conditions, howeveigf the presence of titanium insuitably controlled amounts is efiective in improving the properties ofthealloy and accelerating the heat treatment. Magnesium in amounts up toabout 0.5 per cent can be added to the alloy with a consequent increaseof strength find hardness but with lessened ductility.

The alloy may be cast in sand or other suitable molds and the subsequentheattreatment may be carried out by any known means, as for example agas fired or an electrically heated furnace. As will appear hereinafter,one is in general permitted considerable variations in pouringtemperatures instead 'of being held strictly to narrow limits.

In addition to the soluble elements, such as copper, magnesium, zinc,and silicon, which are added to aluminum alloys for the purpose ofmaking them susceptible to improvement by heat-treatment, relativelyinsoluble elements have been added, such as manganese and chromium, forexample. These elements form hard intermetallic compounds with thealuminum, which when disseminated throughout the aluminum matrix producesome further hardening of the alloy, although the apparently have noparticular effect upon the susceptibility of the alloy toheat-treatment. Titanium, which is quite insoluble, has been previouslyused in aluminum alloys for its hardening effect, just as have theelements iron, manganese and chromium. In my invention, however, thefunction of the titanium is quite different from that of thehardeningelements just referred to. The amount employed is small and its effectis not only different but is out of all proportions to the amount added.Its

presence apparently produces a solidi ficationstructure which makes thecasting peculiarly susceptible to heat-treatment, perhaps by causing thecopper and other soluble constituents to take a form in which they gointo solid solution more readily; and as before stated, the effect is toa large extent independent of the temperature at which the alloy ispoured and the thickness of the cast section.

As an example of the. invention, the following table shows the tensilestrength and elongation of sand-cast half-inch cylindrical test bars ofalloy having the composition: copper, 4.4 per cent; iron, 0.7 per cent;silicon. 0.7 per cent, and titanium 0.23 per cent. For comparison thetable gives the tensile strength and elon ation of similar bars of thesame alloy .wit out titanium. The bars were poured at a temperature of1400 F. (760 0.), then heat-treated at 960 F. (515 C.) forthe indicatedperiods, quenched and aged at 212 F. (100 C.) for one-half our.

wan 0.23%tltlnlum Without titanium Toma Elonaatktm Tomm Elonntiqn lbsJaqdn 131$... Ilia/sq. m. m ''nih m 36,376 8.0 D, 140 5.5 34,8) 10.0 ,m 6.035, 10.5 31, 6.8 aaoao 10.2 new 1.8

' It is seen from the above that substantially maximum properties of thetest bars containing 0.23 per cent of titanium were developed after onlythree hoursi; high temperature treatment, whereas without the titaniumthe properties were less satisfactory, even after eating for thirtyhours.

. tures of about 212 F. (100 C. to 300 F..

. In the case of bars of similar composition but cast in 1% inchsections instead of inch sections, the superiority of the barscontaining titanium was especially marked. After heating for threehours, the bar with 0.2 per cent added titanium was about 4120 poundsstronger per square inch, and after thirty hours heating (followed byquenching and aging as before) was still 3300 pounds stronger than thebar without titanium, though in neither case was the tensile strengthper unit cross section as high as in the inch sections. Examination ofthe polished cross-section of these 1% inch bars showed markeddifferences in structure between those with and those without addedtitaniunL, The former were sound and substantially free from the minutecavities expected in thick sand-cast sections of the alloy, while suchcavities were present in large numbers in the bar poured withouttitanium. Even with only four hours heating.

the titanium-containing bars .were as nearly free from undissolvedintergranular CuAl as were the others after much longer treatment.

Even when poured at 1250 F. (675 (1.), the alloy with added titaniumshows its superiority, especially in the thicker section, as forexample, the 1% inch bars just cited. In this case bars with addedtitanium were 4,000 pounds stronger per square inch after four hoursheating, quenching and a ing, and

were still 2,000 pounds stronger a ter thirty.

hours heat treatment.

The presence of the titanium makes it possible in some cases to obtainuseful improvement of physical properties with lower temperatures inheat treatment than heretofore, for example 925 F. (495 C.). It isgenerally preferable, however, to heat-treat at hi her temperatures, sayfrom 950 F. (510 C. to 970 F. (520 (1), but for only a short period ascompared with the time required for castings of the same aluminum-copperalloy without titanium. After heating and quenching or other rapidcooling, the casting may be allowed to age naturally, or it may be agedartificially by heating at tempera- 150 C.) in the usual way.

Although the advantages of my invention are apparent even when the ironcontent of the alloy is relatively high, as evidenced by the resultsexhibited in the table, still more improvement in the physicalproperties of the casting can be obtained if the iron content islessened. For instance, sand-cast test bars A; inch in diameter werecast at 1400 F. (760 C.) from an alloy of the following composition:copper, 5.1 per cent; iron, 0.15 per cent; silicon, 0.15 per cent, andtitanium 0.22 per cent. Similar bars were cast from an alloy of the samecomposition but without the titanium. The latter bars showed a tensilestrength of only 28,300 pounds per square inch and an elongation of only2.8 percent in 2 inches, after heating for twenty hours at a temperatureof 960 F. (515 0.), quenching, and aging for two days at roomtemperature. 011 the other hand, the titanium-containing bars showed atensile strength of 36,820 pounds per square inch and an elongation of5.5 per cent in 2 inches after heating for only five hours, quenchingand aging, and a tensile strength of 42,200 pounds per square inch andelongation of 8.2 per cent after twenty hours heating, quenching andaging.

Although my invention is particularly directed to the production ofaluminum-copper alloy castings and their heat treatment, nevertheless itis not necessarily limited thereby. The alloy can be'rolled, forged orotherwise worked as circumstances may require.

I claim 1. Process of producing articles of aluminum-copper alloy,comprising incorporating in the molten alloy at an appropriate.

stage of its production, titanium in amounts sufiicient to acceleratesubsequent heat treatment, casting the alloy, and heat-treating thecasting under conditions sub-normal. for a similar casting of a similaralloy devoid of titanium.

2. In a process of producing'aluminumcopper-titanium alloy castingscontaining between about 3 and 5.5 per cent copper and 0.05 to 0.5 percent titanium, the steps comproduced by incorporating titanium in themolten alloy at an appropriate stage of its production, casting thealloy and heat treating the casting under conditions subnormal for asimilar casting of a similar alloy devoid of titanium. Y

1'11 testimony whereof I hereto afiix my signature.

ALADAR PACZ.

